Volvo 819 E - Page 36

Group 28 Ignition sys t ems
De sign and fun ct ion - Compensation fu nc ti ons
Q' 0
0' 0
41
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REX-I
1. 1011
Rex -I
On th e Rex-I syst em, the program functions which con trol the tendency of the engine to kn ock are somewhat di ffer -
ent to tho se in the EZ-K syst ems . The differences are des c ribed in the following pages .
Knock characteristic
The figure shows the 'no rmal ' ignition se tt ing (0.) assuming, in th is case, that speed and load are cons ta nt (hence the
extremely simplified 's traight ' yellow line). The vertical axis ind icates the extent to wh ich th e timing has been re tard-
ed in relation to the normal se tt ing, 0. The horizontal axis indicates the tim e, as defin ed by a certain nu mber of en·
gine revolutions . Knock re ta rd ation is illu strated by the downward steps (red) and re ·advance by the sma ller upward
steps upwards (blue). The maximum retardation is 15 °, here deno t ed by '0 - 1 5' .
Knock-controlled retardation
When the engine is running, the control unit co mputes the optimum igni tion t iming on the basis of speed and load
signals. The occurrence of knock in a cylinder is detected by the control uni t, wh ich retards th e ignition by a step of 3"
in the cylinder in question . If the phenomenon persists, the con trol unit re tar ds the ignit i on by a further step of 3°
In -6") and so on, until knock is e li minated .
Ae-advance at different rates
Re-advance of the i gnit i on to the normal sett i ng (a), or until knock again occurs, takes place at di fferent rates depend-
ing on engine speed. However , the steps are equal in magnitude in all cases, representing an advance of 1° at a time.
Once
the ti ming has been retarded ( to a -JO or morel (a ), the succeedi ng re-advance steps (b) are performed tw ice as
fast. Thus , the interval 'a' in the fi gure is tw ice as long as 'b '.
At high eng ine speed, the interval 'a' dur ing which t he tim ing is retarded ( to a -3" or more) is relat ively long . (The in-
terval is e
qu i valent to approx. 80 revolutions at an engine speed of abou t 6000 rImin).
At
low eng ine speed, the corresponding interval is abou t half that at high speed lapprox . 35 revolutions at an engine
speed of about 1000 rlmin }.
The re -advance rates vary across the speed range, becom ing slo wer as e ngine speed increases.
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